This invention relates to ignition coils for internal combustion (IC) engines, both capacitive discharge (CD) and inductive. In particular, it relates to modern ignition coils with segmented bobbins in which the secondary winding is wound in separate bays, and more particularly to high efficiency ignition coils, both CD and inductive, which have a low secondary resistance typically in the 100 to 500 ohm range and use spark plug wires and spark plugs with direct current (DC) resistance substantially less than 1000 ohms. The invention addresses issues of transient high voltage resulting from ignition spark firing of such high efficiency coils, in particular issues associated with the high voltage spark discharge as it reflects itself in the ignition coil high voltage xe2x80x9cend-effectsxe2x80x9d and electromagnetic interference (EMI). In addition, this invention relates to systems for fabricating and encapsulating such high efficiency coils with the improvements made to resolve the end-effect and EMI issues consistent with a coil structure that is not susceptible to high voltage corona discharge or to cracking from temperature variations which may result from the designs.
Current CD and inductive ignition systems are very inefficient, with secondary winding resistance in the thousands of ohms, and typically using spark plug wire with resistance in the 5,000 to 10,000 ohms per foot, and resistive spark plugs which typically have resistance of about 5000 ohms. These high resistance values serve to reduce EMI associated with the spark firing, which occurs when the various ignition secondary circuit capacitances discharge from their initial high voltages of typically 8 to 24 kilovolts (kV), to close to ground potential when the spark is formed. In addition, the high secondary resistance allows the voltage at the ignition coil high voltage tower to decrease relatively slowly and smoothly so that the voltage distribution across the coil secondary windings following spark firing is relatively smooth and low. In particular, in the case of a modern segmented bobbin with a total wire plus plug resistance of 10,000 to 20,000 ohms, the voltage drop across the last bay is limited to a small fraction, typically ⅕ to ⅓ the coil peak output voltage Vs which, with proper design and high voltage isolation margins for the more limited peak voltage Vs, will not cause breakdown across the last bay of the coil to damage the coil.
On the other hand, for very high efficiency ignition systems, the voltage across the last bay of the coil secondary winding can be equal to and even greater than the voltage Vs. Two such very high efficiency ignitions are the inductive 42 volt based ignition disclosed in my prior patent application PCT/US96/19898, filed Dec. 12, 1996, (WO 97/21920. Jun. 19, 1997 publication date), and the CD based ignition disclosed in my U.S. Pat. No. 5,947,093, issued Sept. 7, 1999. Their secondary winding resistances are very low, typically 100 to 200 ohms for the CD version, and about 500 ohms for the inductive version, with the constraint that non-resistive spark plugs are used as well as low resistance inductive suppressor wire with DC resistance typically less than 50 ohms per foot for the CD system and less than 500 ohms per foot for the inductive system.
Such high efficiency coil structures are susceptible to electrical breakdown across their last secondary winding bay upon ignition firing at a preferred high breakdown voltage of 30 to 40 kV. In fact, the coil may survive open circuit peak voltages Vs of 42 kV (where the end effect is not present or diminished), and fail by electrically breaking down across the last bay at a lower breakdown voltage following spark formation. Associated with the spark breakdown in such high efficiency ignition structures is higher than normal EMI and greater susceptibility to corona if the entire coil is not encapsulated given the preferred higher peak output voltages Vs of approximately 42 kV, as disclosed in my previous U.S. Provisional Patent application No. 60/142,008, filed Jul. 1, 1999. However, it is not industry practice to encapsulate the entire coil structure because of the large differences in expansion coefficient between the magnetic laminations and copper wire and the bobbin and the encapsulant (usually an epoxy).
This patent application discloses method and apparatus for reducing, to acceptable levels, the ignition coil high voltage spark firing end-effect found in high efficiency ignition systems and for reducing the associated EMI. With such method and apparatus is also disclosed modifications made to the secondary winding bobbin, of a segmented bobbin design, to further reduce the end-effect. Also disclosed is complete ignition coil structures, both CD based and special high efficiency 42 volt based inductive, incorporating such improvements in the form of complete encapsulated structures designed to also minimize high voltage corona and to be less susceptible to cracking under temperature extremes.
One aspect of invention is the discovery/conclusion that with each inductive bay winding xe2x80x9cLixe2x80x9d in a segmented bobbin there is associated a response time xe2x80x9cTrixe2x80x9d, which I define as the time it takes for the voltage across a given bay, particularly across the last or xe2x80x9cnthxe2x80x9d bay, to respond to a sudden change in peak output voltage Vs of the coil (going from a voltage Vs to ground in the order of a few nanoseconds). This response time Tri is typically in the range of 100""s of nanoseconds (nsec), representing a frequency of the order of a few MegaHertz (MHz), and is a function of the number of turns in the bay, among other things. Having drawn the conclusion on Tri, one embodiment of the invention includes a special high dissipation, low capacitance inductor xe2x80x9cLendxe2x80x9d located at the high voltage end of the coil that slows the discharge of the coil output capacitance Cs upon spark firing to a long discharge time period xe2x80x9cTcxe2x80x9d, significantly longer than Trn (Tri for the last bay). The design of the last few bays is preferably modified (e.g. relatively fewer turns) to further accentuate the difference between the times Tc and Trn, giving the voltage across the last bay more time to track the change in voltage Vs(t) to minimize the voltage difference xcex94Vsn(t) across the last bay (with acceptably low voltage differences across each of the next two adjacent bays). In this way, the voltage difference across the last bay (and its adjacent one or more bays) is reduced to a level that will not cause electrical breakdown across the bays.
Another aspect of the invention is the design of a high dissipation, low capacitance inductor Lend for reducing the high voltage end-effect by slowing down and attenuating the discharge of the high voltage coil output capacitance Cs (upon spark firing). Preferably, the inductor has an inductance Lend in the range of 1 to 10 mH, depending on the coil structure it is used with, and uses magnetic core material that is low dissipation below 50 kHz and high dissipation in the range of 200 kiloHertz (kHz) to 2 MHz, e.g. Philips ferrite material E5, E6, E7, E25, Fair-Rite material 75, 76, 77, etc. A preferred embodiment of the inductor Lend is a coaxial inductor with open ends with a well insulated single layer winding of wire over an inner core of typical diameter between xc2xc and xc2xd inches surrounded by an outer tubular magnetic core of typical outer diameter of xc2xd to 1 inch. The inductor preferably uses 100 to 500 turns of magnet wire of American Wire Gauge (AWG) between 30 and 40 with heavy insulation, preferably Teflon, wound over a length of 1 to 6 inches for convenient location either integrated within the coil high voltage tower or located externally on top of the high voltage tower. The inductor may use several different materials for the inner and outer magnetic cores.
The end-effect suppression inductor Lend performs two functions during the spark breakdown: 1) it presents a sufficiently high inductance and sufficiently low shunt capacitance to limit the amplitude of the high frequency components associated with the spark breakdown and slow the discharge of the coil output capacitance Cs to a period of about 1 microsecond or greater, and 2) it provides sufficient dissipation to limit, or essentially eliminate, the overshoot of the high voltage discharge of the output capacitance Cs.
Preferably, the inductor Lend is located within the high voltage tower, preferably at right angles to the axis of the coil bobbin to provide the maximum length of inductor consistent with maintaining a compact coil structure. This is particularly suitable for the 42 volt based inductive ignition already mentioned (patent cited) where the inductor is conveniently placed at the open end of the open magnetic E-core of the coil. Preferably, for both CD and inductive designs, the entire coil, including most of the lamination structure, is encapsulated to minimize chances for high voltage corona breakdown due to the preferred higher voltage operation of the coil, i.e. firing at up to 36 kV versus the typical 24 kV for industry coils. The encapsulation, e.g. epoxy, is preferably highly filled with low expansion coefficient material, e.g. alumina powder, to reduce the expansion coefficient to under 30 PPM/xc2x0C. (parts per million per degree Celsius), and to maximize the thermal conductivity. Also, preferably low expansion coefficient bobbin material is used, such as General Electric Noryl (modified phenylene oxide) with 30% glass filing. Alternatively, the secondary winding may be wound using a universal winding machine to produce free standing separate sections (called pi windings).
Preferably, low resistance, high frequency, inductive suppression spark plug wire is used of resistance under 100 ohms/foot, preferably about 10 ohms/foot, with an inductance of about 100 microHenries (uH)/foot achieved by winding 32 to 38 AWG copper wire on a magnetic core (typically ferrite) of at least 0.1xe2x80x3 diameter. This offers good EMI suppression, with minimum efficiency loss of the coil, in the range of 10 to 100 MHz, where the end-effect inductor Lend is not normally effective due to its stray (shunt) capacitance.
As used herein, the term xe2x80x9caboutxe2x80x9d means between 0.5 and 2 times the value it qualifies and xe2x80x9capproximatelyxe2x80x9d means within xc2x125% of the term it qualifies.
For ignition coils used in a 42 volt based inductive ignition which preferable use coils with open magnetic cores, especially of an open-E type core, a preferred design for the preferred high voltage segmented bobbin is to have the bobbin extend beyond the open end of the magnetic core such that the last few flanges, e.g. two or three flanges can be made of larger diameter, since they are not confined by the two outer legs of the open-E core. Preferably the slot or bay width of these is made much narrower to keep the number of turns relatively low to the previous bays. The result will be lower end-effect voltages across these last few bays and a deeper winding (winding height) to handle the expected higher voltage.
As complete systems, my high efficiency CD and 42 volt based inductive ignitions represent the highest energy, highest efficiency, and lightest weight of all known ignition systems, and are now improved to also provide the highest peak output voltage Vs for a given size coil and ignition system efficiency, with the lowest EMI when used with good, low resistance inductive suppression spark plug wire, and with capacitive spark plugs with inductive suppressors. As complete ignition systems, they also provide the most effective ignition spark, with flow-resistant peak spark current of 300 to 600 ma, spark energy of about 100 millijoules (mJ), with battery to spark efficiency of 50% to 60%, and now with peak output voltage of approximately 42 kV without fear of breakdown due to end-effect.